1. Field of the Invention
The present invention relates to a sensor signal circuit and a measuring instrument.
2. Description of Related Art
There has been known a sensor signal circuit including a drive unit, a sensor for detecting a driving state of the drive unit and outputting a sensor signal, and a controller for driving and controlling the drive unit according to the sensor signal from the sensor, and a measuring instrument using this sensor signal circuit therein has been practically used.
As a measuring instrument 1 as described above, there has been known a contour measuring instrument including, as shown in FIG. 6, a probe 2 for scanning a surface of a workpiece W, a device body 3 for driving this probe 2, a computer circuit 5 for controlling the device body 3, and a cable 6 for connecting the computer circuit 5 to the device body 3.
The device body 3 includes a Y-axis slider 32 provided on a table 31 for sliding the workpiece W in the direction Y (in the vertical direction in a page showing FIG. 6), a Z-axis drive shaft 33 provided in the upright state on the table 31, a Z-axis slider 34 slidably provided along the longitudinal direction of the Z-axis drive shaft 33, and an X-axis slider 35 provided to the Z-axis slider 34 and capable of sliding in the direction X, and the probe 2 is provided to the X-axis slider 35.
There are provided a Z-axis linear encoder for detecting a travel of the Z-axis slider 34, an X-axis linear encoder for detecting a travel of the X-axis slider 35, and a Y-axis linear encoder for detecting a travel of the Y-axis slider 32.
The probe 2 includes a stylus arm 21 provided substantially in parallel to the X-axis direction with one edge side thereof supported by the X-axis slider 35, and a stylus 22 as a measuring element provided at the other edge side of the stylus arm 21 along the Z-axis direction and contacting a surface of a workpiece. The stylus arm 21 is supported by the X-axis slider 35 so that the stylus arm 21 can move along a small arc on the X-Z plain, and a displacement detector (not shown) for detecting oscillation of the stylus arm 21 inside the X-axis slider 35.
The computer circuit 5 and the device body 3 are connected to each other with the cable 6, and a driving speed (a control target) of the X-axis slider 35 is controlled according to a control instruction from the computer circuit 5.
FIG. 7 shows an arrangement of a circuit as a voltage signal transmitting section provided inside the Z-axis slider 34 for driving the X-axis slider 35 and also arrangement of the computer circuit 5 as a controller which is a voltage signal receiving section. The circuit provided inside the Z-axis slider 34, computer circuit 5, and cable 6 form a sensor signal circuit.
An inside portion of the Z-axis slider 34 includes a motor 41 as a drive unit for driving the X-axis slider 35, a tacho generator 42 as a sensor for detecting the revolution speed of the motor 41 and generating a voltage proportional to the revolution speed of the motor 41 as a sensor signal. The tacho generator 42 is a sensor as a revolution detector for detecting revolution of the motor 41 and is also a generator for generating power according to revolutions of the motor 41. The motor 41 is a coil motor for rotating a rotator.
The computer circuit 5 includes an interface circuit 51 (T.G. I/F circuit) for receiving a voltage value of the sensor signal from the tacho generator 42, a comparator 55 for comparing a voltage value from the interface circuit 51 to a specified voltage value input from the outside and specifying the revolution speed of the motor 41 to output the difference, a compensation circuit 56 for compensating characteristics of an output from the comparator 55, and a motor drive circuit 59 as a drive unit for driving the motor 41 according to an output from the compensator circuit 56.
The compensation circuit 56 includes an integration circuit 57 for performing integral compensation on an output from the comparator 55 for compensation, and a PWM circuit 58 (Pulse Width Modulation circuit) for shaping a waveform of an output from the integration circuit 57.
The computer circuit 5 and inside circuit of the Z-axis slider 34 are connected with the cable 6. The motor drive circuit 59 and the motor 41 are connected to each other with a transmission line 61 for transferring an instruction from the motor drive circuit 59 to the motor 41. The tacho generator 42 and the interface circuit 51 are connected to each other with a transmission line 62 for transferring the voltage from the tacho generator 42 to the interface circuit 51.
Operations of the measuring instrument 1 having the arrangement as described above will be described below.
First, a workpiece W is placed on the Y-axis slider 32, and the Z-axis slider 34 is displaced so that the stylus 22 is contacted to a surface of the workpiece. A measuring speed for measurement is input into the computer circuit 5. Namely, a driving speed of the X-axis slider 35 is it into the computer circuit 5. The input driving speed of the X-axis slider 35 is converted to a revolving speed of the motor 41, and further to a voltage value corresponding to the revolving speed of the motor 41, and is input as a specified voltage value to the comparator 55.
The specified voltage value input into the comparator 55 is transmitted through the transmission line 61 of the cable 6 via the compensation circuit 56 and motor drive circuit 59 to revolve the motor 41. When the motor 41 starts revolving, the X-axis slider 35 is driven. When the X-axis slider 35 is driven, the stylus 22 is moved for scanning in the X-axis direction along the surface of the workpiece, and the stylus 22 is displaced in the Z-axis direction along irregularities on the surface of the workpiece. Displacement of the stylus 22 is delivered as oscillation of the stylus arm 21 and is detected by a displacement detector inside the X-axis slider 35. In this step, contour of the surface of the workpiece can be measured by sampling a position of the Z-axis slider 34, a position of the X-axis slider 35, and an oscillation rate of the stylus arm 21. Further, by sliding the Y-axis slider 32, the entire surface of the workpiece can be measured.
The revolution speed of the motor 41 is detected by the tacho generator 42, and a voltage (sensor signal) corresponding to the revolution speed of the motor is output. The voltage from the tacho generator 42 is transmitted through a transmission line 62 of the cable 6 and is received by the interface circuit 51. The voltage value received by the interface circuit 51 is sent to the comparator 55. The comparator 55 compares the specified voltage value to the voltage value from the interface circuit 51, and feeds back the difference via the compensation circuit 56 and motor drive circuit 59 to the motor 41. Then the revolution speed of the motor 41 is controlled at a constant value according to this feed-back information.
By constantly controlling the revolution speed of the motor 41, a driving speed of the X-axis slider 35 can be controlled at a constant value. Then a scanning speed of the stylus 22 can be controlled at a constant value, so that measurement values can be stabilized.
With the sensor signal circuit and measuring instrument 1 using the same, however, the following problems occur.
Inside of the Z-axis slider 34 and the computer circuit 5 are connected with the cable 6. Sometimes electromagnetic coupling based on a combination of an electric field from the outside and a magnetic field may occur inside the cable 6, and noises are mixed in a transmitted signal (voltage).
In FIG. 8, the an transmission line 62 is equivalently replaced with a linear element (such as a resistor or a capacitor) to show influences by an electric field over the transmission line 62 from the tacho generator 42 to the interface circuit 51. When an influence by an external electric field occurs in the transmission line 62 shown in FIG. 8, noises due to the capacitive coupling are generated in the transmission line 62.
To remove the noises, it is conceivable to provide a low-pass filter 52 in the interface circuit 51 as shown in FIG. 8. In FIG. 8, further a noninverting amplifier 53 for amplifying an output from the low-pass filter 52 is provided. Noises can be cut off to some extent by the low-pass filter 52 provided in the inter circuit 51. However, to sufficiently reduce noises, a cut-off frequency in the low-pass filter 52 must be set at a low value. When the cut-off frequency is set at a too low value, control over a motor is disabled, so that noises can not be removed only with the low-pass filter 52.
Further, when a voltage transmitted over the cable 6 is small sometimes a voltage transmitted as a signal may disadvantageously be affected by an electric field from the outside to become ambiguous. When the motor 41 is revolved at a low speed, a voltage output from the tacho generator 42 is small. For instance, if the revolution speed of a motor is 4 rpm, a voltage output from the tacho generator 42 is in the range from 2 mV to 4 mV. When a voltage output from the tacho generator 42 is small noises are generated in the cable 6, and the revolution speed of the motor 41 can not correctly be delivered to the computer circuit 5. In this case, control over the revolutions of the motor 41 can not normally be carried out, so that the revolution speed of the motor may disadvantageously drop or become unstable.
As a method of increasing a voltage delivered as a sensor signal output from the tacho generator 42 even when the motor 41 revolves at a low speed, an arrangement is conceivable, in which a gear is provided between the motor 41 and the tacho generator 42 to differentiate the revolution speeds of the motor 41 and the tacho generator 42. However, since there is a limit for the revolution speed of the tacho generator 42, the maximum revolution speed of the motor 41 must be reduced. When the maximum revolution speed of the motor 41 is limited, a driving speed of the X-axis slider 35 is limited, so that the driving speed disadvantageously drops. For instance, when the maximum scanning speed of the X-axis slider 35 is limited to about 5 mm/s, a long period of time is required for measurement, which lowers the measurement efficiency.
When the problems as described above are taken into consideration, the revolution of the motor 41 cannot be controlled in a wide range from low revolution speed to high revolution speed. Namely, a measuring speed of the measuring instrument 1 can be set only within a narrow range.
Also there is the following problem.
Because of the transmission line 62 in the cable 6, a reference electric potential for the tacho generator 42 is the same as that for the interface circuit 51, so at the voltage is delivered between the tacho generator 42 and the interface circuit 51 under this reference electric potential. However, when noises come into the cable 6 from an external electric field, potential difference is generated between the interface circuit 51 and the tacho generator 42, namely, the both do not have the same reference voltage. In this case, when a reference electric potential for the tacho generator 42 is higher, a voltage of the sensor signal output from the tacho generator 42 is at a value including this reference electric potential. The voltage of this sensor signal is transmitted over the transmission line 62, and when the interface circuit 51 receives the voltage value for this sensor signal, the interface circuit 51 receives the voltage as a voltage measured from a low reference electric potential. Namely, the interface circuit 51 receives the voltage as a voltage (potential difference) higher than the voltage output from the tacho generator 42 (potential). Then, the voltage received by the interface circuit 51 indicates a higher revolution speed as compared to the revolution speed of the motors 41 detected by the tacho generator 42. When the voltage received by the interface circuit 51 is input to the comparator 55 and subtracted from the specified voltage value, a voltage fed into the motor 41 has a value smaller than a desired value. Then, the revolution speed of the motor 41 becomes lower than a predetermined revolution speed. When the potential difference is generated in the transmission line 62 by the external electric field as described above, the revolution speed of the motor detected by the tacho generator 42 can not correctly be fed back to the computer circuit 5. As a result, the revolution speed of the motor 41 can not accurately be controlled.
When the device body 3 and the computer circuit 5 are provided with a large distance therebetween, length of the cable 6 for connecting the computer circuit 5 to the Z-axis slider 34 is required to be long. When a position of the measuring instrument 1 in a plant is taken into consideration, there should be a freedom in the distance between the device body 3 and the computer circuit 5 for convenience, and for instance, the freedom of the length is desired to be around six meters. However, the longer the cable 6 is, the larger the influence by the external electric field becomes. When the influence by the external electric field is large, transmission of a signal with a lower voltage becomes more difficult because of noises, so that a larger potential difference is generated between the tacho generator 42 and the interface circuit 51. Because of the noises generated in the cable 6, it is difficult to lengthen the cable 6. Therefore, there occurs the problem that the device 3 and the computer circuit 5 can not be provided with a large distance therebetween.
The problem as described above is not limited to the measuring instrument 1 (contour measuring instrument) described above, but is common to a sensor signal circuit including a voltage signal transmitting section having a sensor for outputting a result of detection as a voltage signal, a cable for transmitting a signal from the sensor, a voltage signal receiving section for receiving the transmitted signal as well as to a measuring instrument using the sensor signal circuit. Especially, the problem described above becomes remarkable when the sensor outputs a result of detection as a voltage waveform.